Conventionally, the image processing apparatus of the above type uses almost the entire effective display period on a display screen as a sampling window for calculating correction data from maximum and minimum values of a luminance signal, and corrects the input video luminance signal for each field or frame, thereby achieving high quality of images.
For example, there is a scheme known as black extension/white extension. In this scheme, minimum and maximum values of the video luminance signal within the effective display period are detected for each field or each frame; the detected maximum and minimum values of the input luminance signal are converted into a maximum value (255 in digital 8-bit processing) and a minimum value (0, in general), respectively, of a dynamic range of a video signal processing system; and all signals between the minimum and maximum values of the input luminance signal are also linearly interpolated. With this, correction can be made so that the dynamic range held by the signal processing system is fully used for any input luminance signal. An example of this is disclosed in Japanese Patent Laid-Open Publication No. 10-248024.
In the above conventional scheme, however, a correction table is calculated based on the maximum and minimum values of the video luminance signal actually detected, and therefore correction may sometimes not be sufficient. For example, in a movie scene most of which is dark for an effective display period, when even one pixel in the scene has a maximum value (255 in 8-bit processing) of a dynamic range of the processing system, correction by extension in a white direction is not performed. This causes a problem that sufficient gray scale representations cannot be made for some displayed images.
For this reason, an object of the present invention is to provide an image processing apparatus and an image processing method capable of performing sufficient gray scale representations irrespective of a displayed image, and especially capable of doing so in accordance with properties of the displayed image even if a width of luminance distribution of a video luminance signal is large.
The above conventional scheme is not meant to be restrictive. In general, when gray-scale correction is performed by correcting an input video luminance signal, color-difference signals (U, V) are also corrected at the same time in order to compensate for changes in how video looks due to the correction of the luminance signal. At this time, in a case where the degree of correction of the luminance signal is too large, if the color-difference signals are corrected based on that degree of correction, the color-difference signals become saturated. That is, after correction, values of the color-difference signals before correction equal to or larger than a predetermined value become maximum among values that can be originally taken by the color-difference signal after correction. Therefore, information about a color difference as to these values is lost. Moreover, when these luminance signal and color-difference signals are converted into RGB signals for image display on a PC display, for example, the RGB signals can become saturated even though these luminance signal or color-difference signals have not been saturated. Also in this case, as with the case of the color-difference signals, information about color difference as to a portion representing saturation of the RGB signals after correction is lost. As a result of the above, the quality of the displayed image is degraded.
Therefore, another object of the present invention is to provide an image processing apparatus and an image processing method in which color-difference signals or RGB signals are not saturated irrespective of the degree of correction of a luminance signal at the time of gray-scale correction.
Furthermore, in the above conventional scheme, when maximum and minimum values for each field are detected, information in a detection WINDOW is evenly sampled. When such scheme is used for detecting the maximum value, if data unrelated to an original image, such as white subtitles in a film, is inserted afterwards into video signals, the data inserted afterwards is detected as the maximum value. With this, a value having a level much higher than that of the original image is determined as the maximum value and, based on this maximum value, the above-described gray-scale correction is performed. Consequently, the amount of extension in a process, such as white extension in which a gray scale is extended in a white direction for correction, is suppressed, and therefore sufficient correction effects cannot be achieved.
Still further, a portion outside a display area in horizontal and vertical directions have a luminance signal of generally approximately 0. If the luminance level of this portion is detected as the minimum value of the image, a level under a level of black of the original image is erroneously determined as black. Consequently, extension of the gray scale in a black direction is suppressed, and therefore sufficient correction effects cannot be achieved.
Therefore, still another object of the present invention is to provide an image processing apparatus and an image processing method capable of detecting a maximum value of an original image so that, when gray-scale correction is performed in which a maximum value of a video luminance signal is converted into a maximum value of a dynamic range of a video signal processing system, for example, sufficient gray-scale correction effects can be achieved even if a signal unrelated to the original image exists in video signals.
Meanwhile, in a case where gray-scale correction is performed on motion pictures, when a characteristic detection signal indicative of characteristics of an image, such as maximum and minimum values within a display area, is detected, it is required to perform contradictory operations: an operation for causing the characteristic detection signal to follow an abrupt scene change and an operation for preventing a fluctuation of the characteristic detection signal due to a subtle scene change. Conventionally, however, it is difficult to satisfy this requirement with high accuracy.
Therefore, still another object of the present invention is to provide an image processing apparatus and an image processing method capable of correcting a maximum value for use in gray-scale correction to an optimal value in accordance with a scene change so as to follow an abrupt scheme change and not to follow a subtle scheme change, when motion pictures are subjected to gray-scale correction, for example, in which a maximum value of a video luminance signal is converted into a maximum value of a dynamic range in a video signal processing system.